[name]Regan[/name],
I completely agree with Freckles regarding the popularity of [name]Celia[/name]. I just don’t see it ever becoming as popular as [name]Madeleine[/name] (and [name]Madeline[/name] and [name]Madelyn[/name]). As she noted, [name]Celia[/name] has actually dropped in popularity by over 100 places over the past ten years, whereas [name]Madeleine[/name] has enjoyed a rather constant place in name popularity over the past decade - around 300. Now, combined with [name]Madeline[/name] and [name]Madelyn[/name], which both show up on the charts around #60, and you’ve got a pretty common name. Spelling variants make any name seem more popular, simply because there are more ways to get to it. You don’t see any of those with [name]Celia[/name], as Freckles noted. Plus, [name]Madeleine[/name] has a built-in nickname that is also quite common, especially with the 1990s increase in the use of [name]Madison[/name] - [name]Celia[/name] doesn’t have a clearly defined nickname option. As far as being a vintage name, you would think [name]Celia[/name] would be on the rise. However, its more common form, [name]Cecilia[/name], isn’t even that common - around 250. I think vintage nature names, like [name]Violet[/name] and [name]Hazel[/name], and comfortable classics, like [name]Matilda[/name] and [name]Ramona[/name], are more likely to climb the charts, especially when these type of names seem to be consistently chosen by celebrities - in fact, all of the four names I listed have been bestowed on a celebrity child!
Okay, I didn’t want to come on here and lecture about name popularity, because at the end of the day, what do I know? I did want to tell you that I think it is a neat connection that both your name, [name]Regan[/name], and your daughter’s name, [name]Celia[/name], are both Shakespearean! Coming from ‘[name]King[/name] Lear’ and ‘As You Like It,’ respectively, both names have a spunky flavor and romantic feel - though you might not see it at first in your own name - that are distinctly quirky classic. So, without further ado, I present to you some prime Shakespearean names that might fit the bill for [name]Celia[/name]'s little sister:
[name]Adriana[/name]
[name]Alice[/name]
[name]Audrey[/name]
[name]Beatrice[/name]
[name]Bianca[/name]
[name]Cassandra[/name]
[name]Diana[/name]
[name]Francisca[/name]
[name]Helena[/name]
[name]Hermione[/name]
[name]Imogen[/name]
[name]Isabel[/name]
[name]Isabella[/name]
[name]Juliet[/name]
[name]Katharina[/name]
[name]Lavinia[/name]
[name]Luciana[/name]
[name]Margaret[/name]
[name]Marina[/name]
[name]Miranda[/name]
[name]Olivia[/name]
[name]Rosalind[/name]
[name]Rosaline[/name]
[name]Viola[/name]
Of those, ones that I think are the most romantic and feminine, as [name]Celia[/name] is, are [name]Diana[/name], [name]Helena[/name], [name]Isabel[/name], [name]Juliet[/name], [name]Katharina[/name], [name]Luciana[/name], [name]Miranda[/name], [name]Rosaline[/name], and [name]Viola[/name]. You mentioned [name]Celia[/name] being light, and we must have different definitions of light ([name]Lily[/name] is “light” for me!), but I think the special thing about [name]Celia[/name] is that it captures a sense of feminine grace and softness coupled with a strong exterior. That’s what I think you need to find in her sister’s name, and I think these fit the bill. Good luck!
[name]Lemon[/name] 
PS And I did want to mention that if you find yourself liking [name]Bella[/name] but are wary of the ever-popular [name]Isabella[/name], you might consider [name]Arabella[/name], a great, underused vintage name with endless romantic, feminine appeal! [name]Mirabelle[/name] and [name]Christabel[/name] are other classic options that might fit the bill, especially [name]Mirabelle[/name] since it begins with an M. AraBeara or MiraBear, anyone?
PPS I know you weren’t necessarily looking for other suggestions, so I’ll end my suggestions with this post. Best of luck to you, [name]Regan[/name]!